In the case, for example, of an alternator pulley, the rim of the pulley (driving part) is driven by the belt and the hub of the pulley which is securely attached to the alternator (driven part). The filtering elastic function (low-pass filter) is generally provided by an elastomer body.
For strongly acyclic transmission systems (significant engine irregularities) or for systems with a highly stressing usage cycle (alternator/starter, for example), the vibratory effects are such that the rubber filtering body does not generally make it possible to provide a satisfactory trade-off between lifespan and filtering efficiency. Indeed, in order to filter correctly, the torsion stiffnesses of the elastomer body have to be low whereas at the same time, the elastomer body must not be deformed too much if its lifespan is to be increased. The result is therefore very often limited lifespans with average filtering efficiencies.
Decoupling pulleys are known, the elastic stiffness of which that is necessary for the filtering is provided by a torsion spring. Decoupling pulleys intended for automobile accessories (alternator) are described notably in the patent applications US 2006/264280, US 2008/108442 and WO 2009/47816.
These pulleys provide a coupling and a decoupling of the spring via a friction system which imposes having the spring work to open, and which mandatorily impose the implementation of a decoupled operating mode incompatible with certain applications, notably the alternator/starter (SAD) pulleys.
The friction coupling/decoupling system is a source of friction, and therefore of wear, which affects the stability of the performance levels over time, and the reliability and the longevity of the product.
Similarly, it is unfavorable to have a spring work alternately to open and close to provide a coupling and a decoupling, namely a clutching and a declutching.